I say necron. But thats probably just my luck . The easiest is by far to me. The undying. I didnt even struggle one bit. That may be because i enjoy going around doing the hunts but still. Also exdeath is close to hardest but i think with the right set up he is easy. Havent played 3 much.

The actualy easiest boss though is (SPOILER) rursan arbiter from type -0 (atleast on the first playthrough) because there is no strategy at all. Atleast yu yevon couldnt be killed by just hitting him normally. You had to kill him with a sort of strategy. I may be remembering the type-0 fight wrong but.. Iirc you just destroy a part with a character. Then they die. And repeat till last character. Boom. Win.